A DIY Phase Change
Material?

The energy that is absorbed by a
material as it turns from a solid to a liquid can be used to store heat energy
for use at a later time in solar heating (or cooling) systems. This
technique is attractive because 1) the heat is stored or returned over a very
small temperature change, and 2) some phase change materials can store a great
deal of heat in a small volume.

Nick Pine posted the following
message in the
alt.solar.thermal news group. It seems like an area that would be
interesting to experiment in, and a phase change heat storage material that
could be made inexpensively would be very helpful in solar heating systems.

George Lane is 75 years old
and used to work for Dow Chemical and has about
150 patents, including US 4,613,444, "Reversible phase change compositions..."
issued on 9/23/1986, now expired, which describes adding a dash of sodium
chloride and potassium chloride and about 2% strontium chloride hexahydrate
to calcium chloride hexahydrate to help ensure phase change stability, ie
lots of cycles, freezing at 26 C and melting at 30. A similar German product
(Doerken DeltaCool24 or 28) in thin plastic pouches costs about 5 Euros/kg,
plus shipping. George says don't heat his mix much above 40 C, because that
will melt the nucleation centers, which might not return until the mix is
brought to 0 F.

George says the containers need to be strong to avoid being scratched or
punctured by the mechanical actions of freezing and thawing. There's a 10%
volume change with phase, so the containers must be elastic or pleated or
have some air fill to reduce the peak pressure. They want to be well-sealed
to avoid losing or gaining water from surrounding air, with plastic walls
at least 0.035 inches thick, but this can be avoided by keeping the air at
an RH of about 35%, with something like the vapor pressure of CaCl2+6H20
at 90 F (a humidistat might open a solenoid valve with a soaker hose inside
a small room with an air heater. We can't add too much sodium or potassium,
saturation is fine, but too much or too little strontium gives the freezing
curve a slope vs a plateau. George says 1-liter soda bottles could work well
in 90 F air at 35% RH, or less. We might look at the water level and top up
1000 bottles every 10 years.

George made calcium chloride hexahydrate from DowFlake and PellaDow road
salt, at about 5 cents a pound (the strontium salt used in road flares
costs twice as much), by adding salt to water (for safety, since this is
exothermic.) The salt starts as a di- or tetrahydrate with desirable
impurities. When I suggested baking it in an oven to drive off the water,
then adding water to double the weight, he said that couldn't be done
without disassociation, but a university chem lab could analyze the water
content by various simple means. I wonder how...

One application might be a house in Calgary, Alberta, where 751 Btu/ft^2
of sun falls on a south wall on an average 18 F December day. It needs
about 144K Btu for 5 cloudy days in a row.

If 1 liter stores 225 Btu, we might have 144K/225 = 640 1-liter bottles
(about 40 ft^3) in a horizontal hexagonal stack in a small room with
an air heater or a south window made from 2 layers of GE HP92W 10 mil
Lexan polycarbonate film enclosing a 7" waterproof glazing cavity filled
with tiny cold soap bubbles (US R20-30) at night and filled with air
during the day, when 600 Btu/ft^2 of sun might enter the room over 6 hours.

If the store recharges in 5 days at 28.8K Btu/day and the bottles have
565 ft^2 of surface with a 1.5 Btu/h-F-ft^2 airfilm conductance so
the room air is 86+28.8K/6h/1.5/565 = 92 F during charging, 600A
= 28.8K+6h(92-20)A/2 makes the window area A = 58.5 ft^2, so an 8'x8'
window might do. An 8'x12' window could also provide some warm air
for the house on an average day.

Know anyone who can turn George Lane's patent into a foolproof DIY
cookbook recipe? This might be sold in a 1-pound kit with enough
strontium salt to treat a 50 pound bag of local calcium chloride.